The long term objective of this proposal is to study the factors which regulate the peripheral tissue uptake of thyroxine (T4), the main secretory product of the thyroid gland, and subsequent 5'-deiodination to 3,5,3'-triiodothyronine (T3), the most potent thyroid hormone, and 5-deiodination to 3,3',5' triiodothyronine (rT3), a thyromimetically inactive product. Specifically, emphasis will be placed on the role of the kidney in the whole body production of T3. Using an established method of kidney perfusion, as well as plasma membrane preparations, the mechanisms of renal uptake and deiodination of T4 and T3 will be characterized. The locus of deiodination will be functionally identified relative to the site of tubular iodide handling in the perfused kidney. Measurements of thyroid hormone metabolites will be accomplished both with radioimmunoassay techniques and with high pressure liquid chromatographic (HPLC) analysis of radioactive tracer compounds. The perfused organ system provides the opportunity to study thyroid hormone metabolism in an intact organ which is separation from the confounding effects of other tissues. In addition, the perfused organ is potentially able to respond to hormonal and substrate stimuli. The mechanisms regulating the decreased renal production during diabetes mellitus will be further explored. In addition, the perfused kidney's production and degradation of rT3 will be compared with its T3 production. The effect of membrane-stabilizing drugs and organic anions on thyroid hormone uptake and metabolism will be examined. Rates of influx and efflux of thyroid hormone will be used to characterize changes in the renal retention of these compounds. Cytosolic-binding will be correlated with changes in organ uptake of iodothyronines after alterations in metabolic state and therapy with certain drugs. Preparations of luminal and contraluminal renal plasma membrane vesicles will be used to study the relative 5'-deiodination rates at these sites as well as to explore the existence of membrane transport systems for iodothyronines and iodide. These studies will contribute to the understanding of individual organ thyroid hormone uptake and metabolism, its relation to thyroid hormone action, and potential steps at which thyromimetic activity may be modified at the cellular level.